It would have covered 200 square kilometres and been up to 450 metres deep, the researchers write in the journal Geophysical Research Letters.

The presence of water on Mars is widely accepted. The Mars Phoenix mission last year found frozen water on the surface of Mars, and there is also evidence that water may still seep to the surface from underground.

Planetary scientists have also seen what could be the shores of giant rivers and seas, but some of the formations could also arguably have been made by dry landslides.

"This is the first unambiguous evidence of shorelines on the surface of Mars," says research associate Gaetano Di Achille, who led the study.

"The identification of the shorelines and accompanying geological evidence allows us to calculate the size and volume of the lake, which appears to have formed about 3.4 billion years ago," says Di Achille.

Key to life

Water is key to life and scientists are using evidence of its existence to look for evidence of life, past or present, on Mars. Having water on the planet could also be useful to future human explorers.

"On Earth, deltas and lakes are excellent collectors and preservers of signs of past life," says Di Achille. "If life ever arose on Mars, deltas may be the key to unlocking Mars' biological past."

"Not only does this research prove there was a long-lived lake system on Mars, but we can see that the lake formed after the warm, wet period is thought to have dissipated," says co-author Assistant Professor Brian Hynek.

No one knows what turned Mars from a warm, wet planet into the frozen, airless desert it is now.

But the researchers suggest that the lake either evaporated or froze over after abrupt climate change, with its waters turning into vapour.